The main components of a motor vehicle drivetrain are a drive assembly and a transmission. The transmission converts torques and rotational speeds, thereby transforming the driving force provided by the drive assembly. The present invention concerns a method for operating a drivetrain which comprises a transmission and, as its drive assembly, a hybrid drive with an internal combustion engine and an electric motor.
In a drivetrain with a so-termed parallel hybrid drive, a clutch is interposed between the combustion engine and the electric motor of the hybrid drive, by means of which the combustion engine of the hybrid drive can be disengaged from a drive output of the drivetrain. When such a drivetrain with a parallel hybrid drive is started up, typically the combustion engine is switched off and the clutch interposed between the combustion engine and the electric motor of the hybrid drive is disengaged, so that starting takes place solely by means of the electric motor of the hybrid drive, ideally from a rotational speed of zero, since the electric motor can already provide a high torque at very low speeds. Accordingly, during such a start of the drivetrain from zero speed there is no so-termed idling speed that could be used to drive a main hydraulic pump associated with the transmission. So in order, nevertheless, to be able to provide a hydraulic pressure in the drivetrain required for starting, in addition to the main hydraulic pump an auxiliary hydraulic pump as well is associated with the transmission, with the help of which a hydraulic pressure required for starting can be produced.